Material selection for weight scale

Question

I am looking to test the FlexiForce sensor in measure force, however for accurate readings the load needs to be disturbed evenly over the sensing area. As such I am having "discs" made to cover the sensing area. For my application the thinner the material of the disc the better (1 mm or less in thickness). Which of the following materials will be most suitable to manufacture the disc for the stiffest/strongest possible part?

1. Brass & Copper
2. Aluminium
3. Stainless Steel
4. Mild/Carbon Steel

The disc will be 25.4 mm in diameter with a thickness of 1 mm.

Here's an illustration of what I mean:

Answer 1

Which of the following materials will be most suitable to manufacture the disc for the stiffest/strongest possible part.

It seems that you need a material that will transform a non-uniform load into a more-or-less uniform load by providing consistent bearing. As such, you're looking at something like a plate on an elastic foundation, albeit on a much smaller scale. This type of problem can be quite complex due to the interplay between the stiffness of the plate and the foundation and the loading on the plate.

The only variables you seem to have locked down are the 1 mm thickness and 25.4 mm diameter of the disc. Strength is not an issue here (assuming you have a ductile material); stiffness is the only real requirement.

If you are truly looking to get the stiffest material for your disc, the carbon steel you have listed is probably your winner, with stainless a possibility depending on the alloy. Assuming all the discs have the same thickness/diameter, the only thing that will then affect stiffness is the material's modulus of elasticity. The greater the modulus, the stiffer the disc.

---

Tungsten carbide might be worth looking into for your disc as well. It's incredibly stiff (twice that of steel) and not terribly expensive.

Answer 2

1) I've used several brands of FSRs, including FlexiForce and you're very unlikely to get results usable in trade; their responses to pressure vary with temperature (because, I suspect, the stiffness of the polymers in the sensor itself vary w/temp.)

2) The application guides I've read strongly suggest NOT using extremely stiff materials or sharp radii in direct contact with the sensor that can cause stress concentrations.

3) The stiffness and shape of both what's on top of the sensor and what's underneath both matter.

4) all of the materials you're considering are orders of magnitude stiffer than the sensor, so they'll likely give you equally wrong and expensive results.

5) I suggest you first try either thin disks or very-slightly rounded domes made with something like Nylon or HDPE (High-density polyethylene, AKA cutting-board plastic.) I suspect you can back those with disks of a more rigid material; steel would be fine.

6) The disk has to be same-size or slightly smaller than the sensor's active area, or else some of your load will bypass the active area, leading to systemic errors. And you'd better radius the edges, lest you create a stress concentration right there.